A hystat transmission is a type of continuously variable transmission having a hydraulic pump connected to a hydraulic motor in a closed loop configuration. The pump is driven by a power source, for example an engine. The motor is connected to a load, for example a traction device of a vehicle. As the engine operates, the pump is driven to pressurize hydraulic fluid. The pressurized hydraulic fluid is passed from the pump through the motor to drive the traction device and thereby propel the vehicle.
The speed and/or force of the motor can be operator controlled by varying a discharge of the pump in response to a received operator input. For example, when an operator depresses an accelerator pedal to indicate a desire for more speed or torque of the traction device, a discharge of the pump (flow and/or pressure) is proportionally increased. To protect components of the transmission from damage, operation of the pump and/or motor is commonly limited according to pressure. That is, when the pressure of the transmission exceeds a predetermined pressure, a valve is opened to relieve the pressure before damage can occur. Also in response to the high pressure, the pump output can be reduced to minimize the amount of fluid being relieved.
An example of a hystat transmission is described in U.S. Pat. No. 6,405,530 (the '530 patent) issued to Brimeyer et al. on Jun. 18, 2002. The '530 patent discloses a variable displacement pump arranged in a closed loop circuit with a hydraulic motor. The hydraulic motor is drivingly connected to the wheels of a vehicle. The pump is an axial piston type pump driven by an engine and having a movable swashplate to vary a displacement and directional output of the pump. The closed loop circuit also includes a servo mechanism movable to control an angle of the swashplate, and a pressure relief valve used to limit a maximum pressure of the circuit.
In operation, when an operator applies an input, a displacement magnitude signal is communicated to the servo piston to move the swashplate a proportional amount. The pressure relief valve establishes the maximum pressure differential of the circuit, and is controlled to selectively permit bypassing for towing or other purposes, as needed.
Although perhaps somewhat effective, this type of control can induce instabilities and reduce efficiency of the engine driving the pump. That is, proportional swashplate modulation of the prior art systems, coupled with the use of a pressure relief valve, can create a time lag exhibited in pressure fluctuations. Specifically, after circuit pressure has exceeded a maximum allowable pressure and the servo piston is moving to reduce swashplate angle, the pressure of the system may still be increasing until the servo piston has moved a significant distance. During this time, the pressure may spike, causing the pressure relief valve to open and relieve the spike in an attempt to minimize damage. When the pressure relief valve opens, the servo piston may still be moving to reduce pressures, even though the pressure relief valve has already opened and relieved the pressure. As a result, the angle of the swashplate can overshoot an equilibrium angle, causing a drop in the pressure. In response to the now low pressure, the servo piston may again be controlled to stroke up the displacement of the pump, and the cycle is repeated until the equilibrium pressure is finally attained. This instability can result in poor system response, while opening of the pressure relief valve can waste pressurized fluid and thereby reduce engine efficiency.
The transmission of the present disclosure solves one or more of the problems set forth above.